In a system controlled by or depending on a visual display it is common to provide touch-sensitive virtual switches or controls for operator interaction with the system. Such controls replace mechanical equivalents such as buttons or levers, and are believed, among other potential advantages, to provide for a more natural interaction between the operator and the system.
One such group of controls or switches change in electrical impedance when touched by the operator's finger. The controls typically include an electrically-conductive layer deposited on an outermost faceplate of a visual display screen. The electrically-conductive layer forms a resistive or capacitive element of the switch. The system is provided with electrical circuitry which senses the impedance change, identifies which switch or control was touched and implements system instructions accordingly.
The electrically-conductive layer is often formed from a transparent electrically-conductive metal-oxide. A group of metal-oxides commonly used for such layers includes indium-tin oxide (ITO), cadmium-tin oxide, antimony-doped tin oxide, and fluorine-doped tin oxide. While such layers are optically transparent they have a relatively high refractive index, which, depending on factors including a method by which they are formed, have a refractive index for visible light between about 1.9 and 2.2. Such a high refractive index imparts a relatively high reflectivity to layers formed from these materials, even when the layer is relatively thin. Because of this, the switches are readily distinguishable from a substrate on which they are formed even when no graphic outline or identification is provided.
In a simple form, such switches may take the form of relatively large (larger than a fingertip) regions which are actually designed to simulate a mechanical switch, button, lever or the like. In this form it is not a disadvantage that the switches are visible, as, in this form, the switch is usually provided with a graphic outline or identification as part of a display design.
In a more complex form, the switches are designed to substitute for the action of mechanical cursor keys or a mouse in controlling a visual (video) display of text or graphic matter. In this form, the switches may take the form of a regular pattern of spaced-apart electrode-regions formed on a cathode-ray tube (CRT) faceplate. The electrode-regions and spaces therebetween have dimensions generally smaller than a fingertip. Size of the electrode-regions and spacing therebetween determines, at least in part, accuracy with which a location of a point of the screen which is touched by an operator can be determined.
This complex form is not often used in a touch-sensitive video display, at least partly because electrode-regions thereof are readily distinguishable as a regular pattern overlaying the display. They are distinguishable because of a different reflectivity value or reflection color of the electrode-regions and intervening spaces. Such a pattern can be distracting to an operator of the video display.
A commonly used touch detection arrangement for a touch-sensitive video display has a transparent electrode extending continuously over the faceplate of the video display, and relies on features such as complex peripheral busbar arrangements and electronic circuitry to determine touch location. While such an arrangement does not create a distracting pattern, it may be less accurate or more subject to error than an arrangement including discrete electrode-regions as described above.